The present invention relates to distributed electronic process control systems having a number of widely spaced control stations interactively communicating in the control of a complex industrial process. More particularly, this invention relates to a method and apparatus for controlling access rights to a multi-master communications channel for such a system.
The control of complex industrial processes has evolved from the use of a large number of simple single loop controllers, which either perform without central direction or alternatively are directed by a central computer, to the use of distributed systems. In distributed systems, widely spaced control stations are connected for communication with one another and with a host computer, and each of the stations may be capable of controlling a large number of loops. The individual stations are usually microprocessor based and a host computer is employed for complex computing, control, and storage functions beyond the capability of the stations.
In complex data communication systems, such as the interacting system of the present invention, an issue always arises as to how traffic conflicts can be avoided when the network shares a common data highway with no central control being exercised over the rights the individual stations have to access to the network communication channels. A large number of different methods exist for arbitrating access rights when no central control of those rights is exercised. These methods include, among others, the Carrier Sense Multiple Access with Collision Detect (CSMA/CD) method and the token passing method. In the CSMA/CD method, if a device wants to transmit, it first listens to find out if any other station is transmitting. The station does not interrupt any other transmission. When the transmission medium is not busy, the station transmits.
With light data traffic on the medium, there is little conflict with the CSMA/CE method. However, with a large number of stations, they will occasionally transmit simultaneously. These occasions can be sensed so as to cause an abort of the transmission, at which time each station will wait a random amount of time before attempting to transmit. A major disadvantage of this system is the increasing uncertainty of data delivery as traffic increases in addition to the limitations which arise because the data rate, data packet size, and the physical length of the cable all depend upon each other.
Token passing, on the other hand, is a generic name for line-access methods where control of the rights to access (holding the token or mastership) are distributed among all the stations using the data highway. Only one station has the right to transmit to the remaining stations (slaves), that right having been granted by the previous holder of the token. After the station finishes transmission, it passes the token to another station. The token is passed from station to station in a predetermined logical sequence.
While the token passing method is more complicated, it provides an increased flexibility which has many advantages. Also, its deterministic nature commends its use in process control systems where the probalistic nature of the CSMA/CD method would present problems due to the necessity for certain control loops to be looked at frequently in order to maintain rapidly changing process variables under control.
The present invention provides an improved token passing method which provides for increased efficiency in data transmission and which assures with greater certainty that critical process loops are looked at with the requisite frequency to accomplish the necessary control of those loops.